1. Technical Field
The present disclosure relates to electrosurgical devices suitable for use in tissue ablation applications and, more particularly, to ablation devices with dual operating frequencies, systems including the same, and methods of adjusting ablation volume using the same.
2. Discussion of Related Art
Treatment of certain diseases requires the destruction of malignant tissue growths, e.g., tumors. Electromagnetic radiation can be used to heat and destroy tumor cells. Treatment may involve inserting ablation probes into tissues where cancerous tumors have been identified. Once the probes are positioned, electromagnetic energy is passed through the probes into surrounding tissue.
In the treatment of diseases such as cancer, certain types of tumor cells have been found to denature at elevated temperatures that are slightly lower than temperatures normally injurious to healthy cells. Known treatment methods, such as hyperthermia therapy, heat diseased cells to temperatures above 41° C. while maintaining adjacent healthy cells below the temperature at which irreversible cell destruction occurs. These methods involve applying electromagnetic radiation to heat, ablate and/or coagulate tissue. Microwave energy is sometimes utilized to perform these methods. Other procedures utilizing electromagnetic radiation to heat tissue also include coagulation, cutting and/or ablation of tissue.
Electrosurgical devices utilizing electromagnetic radiation have been developed for a variety of uses and applications. A number of devices are available that can be used to provide high bursts of energy for short periods of time to achieve cutting and coagulative effects on various tissues. There are a number of different types of apparatus that can be used to perform ablation procedures. Typically, microwave apparatus for use in ablation procedures include a microwave generator that functions as an energy source, and a microwave surgical instrument (e.g., microwave ablation probe) having an antenna assembly for directing the energy to the target tissue. The microwave generator and surgical instrument are typically operatively coupled by a cable assembly having a plurality of conductors for transmitting microwave energy from the generator to the instrument, and for communicating control, feedback and identification signals between the instrument and the generator.
There are several types of microwave probes in use, e.g., monopole, dipole and helical, which may be used in tissue ablation applications. In monopole and dipole antenna assemblies, microwave energy generally radiates perpendicularly away from the axis of the conductor. Monopole antenna assemblies typically include a single, elongated conductor. A typical dipole antenna assembly includes two elongated conductors that are linearly-aligned and positioned end-to-end relative to one another with an electrical insulator placed therebetween. Helical antenna assemblies include helically-shaped conductor configurations of various dimensions, e.g., diameter and length. The main modes of operation of a helical antenna assembly are normal mode (broadside), in which the field radiated by the helix is maximum in a perpendicular plane to the helix axis, and axial mode (end fire), in which maximum radiation is along the helix axis.
The particular type of tissue ablation procedure may dictate a particular ablation volume in order to achieve a desired surgical outcome. Ablation volume is correlated with antenna design, antenna performance, antenna impedance, ablation time and wattage, and tissue characteristics, e.g., tissue impedance.
In treatment methods utilizing electromagnetic radiation, such as hyperthermia therapy, the transference or dispersion of heat generally may occur by mechanisms of radiation, conduction, and convection. “Thermal radiation” and “radiative heat transfer” are two terms used to describe the transfer of energy in the form of electromagnetic waves (e.g., as predicted by electromagnetic wave theory) or photons (e.g., as predicted by quantum mechanics). In the context of heat transfer, the term “conduction” generally refers to the transfer of energy from more energetic to less energetic particles of substances due to interactions between the particles. The term “convection” generally refers to the energy transfer between a solid surface and an adjacent moving fluid. Convection heat transfer may be a combination of diffusion or molecular motion within the fluid and the bulk or macroscopic motion of the fluid.
The extent of tissue heating may depend on several factors including the rate at which energy is absorbed by, or dissipated in, the tissue under treatment. The electromagnetic-energy absorption rate in biological tissue may be quantified by the specific absorption rate (SAR), a measure of the energy per unit mass absorbed by tissue and is usually expressed in units of watts per kilogram (W/kg). One method to determine the SAR is to measure the rate of temperature rise in tissue as a function of the specific heat capacity of the tissue. This method requires knowledge of the specific heat of the tissue. A second method is to determine the SAR by measuring the electric field strength in tissue. This method requires knowledge of the conductivity and density values of the tissue.
The relationship between radiation and SAR may be expressed as
                              SAR          =                                                    1                ⁢                σ                                            2                ⁢                ρ                                      ⁢                                                          E                                            2                                      ,                            (        1        )            where σ is the tissue electrical conductivity in units of Siemens per meter (S/m), ρ is the tissue density in units of kilograms per cubic meter (kg/m3), and |E| is the magnitude of the local electric field in units of volts per meter (V/m).
The relationship between the initial temperature rise ΔT (° C.) in tissue and the specific absorption rate may be expressed as
                                          Δ            ⁢                                                  ⁢            T                    =                                    1              c                        ⁢            SAR            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            t                          ,                            (        2        )            where c is the specific heat of the tissue (in units of Joules/kg-° C.), and Δt is the time period of exposure in seconds (sec). Substituting equation (1) into equation (2) yields a relation between the induced temperature rise in tissue and the applied electric field as
                              Δ          ⁢                                          ⁢          T                =                                            1              ⁢              σ                                      2              ⁢              ρ              ⁢                                                          ⁢              c                                ⁢                                                  E                                      2                    ⁢          Δ          ⁢                                          ⁢                      t            .                                              (        3        )            
As can be seen from the above equations, modifying the local electric-field amplitude directly affects the local energy absorption and induced temperature rise in tissue. In treatment methods such as hyperthermia therapy, it would be desirable to deposit an electric field of sufficient magnitude to heat malignant tissue to temperatures above 41° C. while limiting the SAR magnitude in nearby healthy tissue to be less than that within the tumor to keep the healthy cells below the temperature causing cell death.
Fluid-cooled or dielectrically-buffered microwave devices may be used in ablation procedures. During operation of a microwave ablation device, if proper cooling is not maintained, e.g., flow of coolant or buffering fluid is interrupted, the microwave ablation device may exhibit rapid failures due to the heat generated from the increased reflected power. Cooling the ablation probe may enhance the overall heating pattern of the antenna and prevent damage to the antenna.
During certain procedures, it can be difficult to assess the extent to which the microwave energy will radiate into the surrounding tissue, making it difficult to determine the area or volume of surrounding tissue that will be ablated. Tissue ablation devices capable of influencing the SAR and the ablation volume may enable more precise ablation treatments, which may lead to shorter patient recovery times, fewer complications from undesired tissue damage, and improved patient outcomes.